<!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <title>Title</title>
    <style>
        .wrapper {
            width: 600px;
            border: 2px solid black;
            margin: 50px auto;
        /*    手动指定高度*/
            height: 300px;
        }


        .box1, .box2, .box3 {
            width: 200px;
            height: 150px;
        }

        .box1 {
            background-color: #FF0000;
            float: left;
        }

        .box2 {
            background-color: blue;
            float: left;
        }

        .box3 {
            background-color: green;
        }
    </style>
</head>
<body>
<div class="wrapper">
    <div class="box1">box1</div>
    <div class="box2">box2</div>
</div>
<div class="box3">box3</div>

</body>
</html>